258 ANIMALS AND PLANTS CHAP. 



contractile vacuole, they are almost universally classed 

 amongst plants, while Monads are as constantly included 

 in the animal kingdom. 



We see, then, that while it is quite easy to divide the 

 higher organisms into the two distinct groups plants 

 and animals, any such separation is by no means easy 

 in the case of the lowest forms of life. It was in recog- 

 nition of this fact that Haeckel proposed, many years 

 ago, to institute a third " kingdom " called Protista, to 

 include all unicellular organisms. Although open to 

 many objections in practice, there is a great deal to be 

 said for the proposal. From the strictly scientific point 

 of view it is quite as justifiable to make three subdivisions 

 of living things as two : the line between animals and 

 plants is quite as arbitrary as that between protists and 

 plants or between protists and animals, and no more so : 

 the chief objection to the change is that it doubles the 

 difficulties by making two artificial boundaries instead 

 of one. 



The important point for the student to recognise is 

 that these boundaries are artificial, and that there are no 

 scientific frontiers in Nature. As in the liquefaction of 

 gases there is a " critical point " at which the substance 

 under experiment is neither gaseous nor liquid : as in a 

 mountainous country it is impossible to say where 

 mountain ends and valley begins : as in the development 

 of an animal it is futile to argue about the exact period 

 when, for instance, the egg becomes a tadpole or the 

 tadpole a frog : so in the case under discussion. The 

 distinction between the higher plants and animals is 

 perfectly sharp and obvious, but when the two groups 

 are traced downwards they are found gradually to 

 merge, as it were, into an assemblage of organisms 

 which partake of the characters of both kingdoms, and 

 cannot without a certain violence be either included in 



